6.4. SUSPENSION-DOMINATED MODELS
311
Shields criterion need not be met in the surf zone because turbulence, not
bed shear, is the dominant cause of sediment mobilization; and therefore,
bed shear is not an important consideration above Reynolds numbers constituting the fully rough range. Dean made specific recommendations for
successful modeling of surf zone processes:
a. The model must be geometrically undistorted (equal horizontal and vertical length scales).
b. Hydrodynamics should be scaled according to Froude similarity.
c. Similarity of the fall speed parameter should be maintained
between prototype and model.
d. The model must be large enough to preclude significant viscous, surface tension, and cohesive sediment effects so that
the character of wave breaking is properly simulated.
e. Sand is preferred as the model material, otherwise lightweight
particles may be used provided the fall speed meets the
scaling criterion.
Fulfilling the above requirements in a sand model satisfies the undistorted
Froude hydrodynamic requirement, as well as preserving the prototype-tomodel scale ratios of wave steepness
/Lo\ fall speed parameter (H/wT),
and relative density (j)s/p).
Dean (1985) argued that, in a geometrically undistorted model, the
fall trajectory of a suspended particle must be geometrically similar to the
equivalent prototype trajectory and fall with a time proportional to the
prototype fall time. This is accomplished by ensuring similarity of the fall
speed parameter between the prototype and the geometrically undistorted
model. Similar fall trajectories also requires that the morphological time
scale be the same cis the hydrodynamic time scale.
Dean noted that similarity of sediment fall trajectory could also be
achieved in geometrically distorted models16, but he did not recommend
distorting movable-bed models because of uncertainties involved if sediment
fall speed is not scaled according to the Froude criterion. An additional
concern is the possibility of dissimilar hydrodynamic surf zone conditions
between prototype and model when distorted scaling is introduced.
Wang, et al. (1990) subsequently derived the scale relationships for geometrically
distorted models that preserve the sediment fall trajectory. This model was discussed in
a previous section.
Dean evaluated his recommendations with intuitive reasoning and by
examination of past results in light of the suggested criteria. He concluded
311
Shields criterion need not be met in the surf zone because turbulence, not
bed shear, is the dominant cause of sediment mobilization; and therefore,
bed shear is not an important consideration above Reynolds numbers constituting the fully rough range. Dean made specific recommendations for
successful modeling of surf zone processes:
a. The model must be geometrically undistorted (equal horizontal and vertical length scales).
b. Hydrodynamics should be scaled according to Froude similarity.
c. Similarity of the fall speed parameter should be maintained
between prototype and model.
d. The model must be large enough to preclude significant viscous, surface tension, and cohesive sediment effects so that
the character of wave breaking is properly simulated.
e. Sand is preferred as the model material, otherwise lightweight
particles may be used provided the fall speed meets the
scaling criterion.
Fulfilling the above requirements in a sand model satisfies the undistorted
Froude hydrodynamic requirement, as well as preserving the prototype-tomodel scale ratios of wave steepness
/Lo\ fall speed parameter (H/wT),
and relative density (j)s/p).
Dean (1985) argued that, in a geometrically undistorted model, the
fall trajectory of a suspended particle must be geometrically similar to the
equivalent prototype trajectory and fall with a time proportional to the
prototype fall time. This is accomplished by ensuring similarity of the fall
speed parameter between the prototype and the geometrically undistorted
model. Similar fall trajectories also requires that the morphological time
scale be the same cis the hydrodynamic time scale.
Dean noted that similarity of sediment fall trajectory could also be
achieved in geometrically distorted models16, but he did not recommend
distorting movable-bed models because of uncertainties involved if sediment
fall speed is not scaled according to the Froude criterion. An additional
concern is the possibility of dissimilar hydrodynamic surf zone conditions
between prototype and model when distorted scaling is introduced.
Wang, et al. (1990) subsequently derived the scale relationships for geometrically
distorted models that preserve the sediment fall trajectory. This model was discussed in
a previous section.
Dean evaluated his recommendations with intuitive reasoning and by
examination of past results in light of the suggested criteria. He concluded